Anti-tumor Agents
While numerous advances have been made in the treatment of cancer, solid tumors remain difficult to treat. Consequently, many patients develop advanced cancer for which conventional therapies are generally not effective. As a result, novel types of cancer drugs are being investigated for anti-cancer activities. In recent years, considerable interest has been generated in platinum coordination compounds (Rosenberg et al., 1969, Nature, 222:385-386). Structurally, they represent a complex formed by a central atom of platinum and surrounded by various arrangements of chlorine atoms and ammonia groups in either a cis or trans relationship. Platinum-based multiagent chemotherapy has become the first line post-surgical therapy for patients with advanced ovarian cancer (American Cancer Society, 1995, Facts and Figures). Cisplatin (cis-diamine dichloro platinum) and Carboplatin (1,1,-cyclobutanedicarboxyl diammine platinum-(II)) are examples of cytotoxic platinum coordination compounds that are useful for treatment of a variety of malignancies. Other platinum compounds known to exhibit cytotoxic effects towards cancer cells include 1,4, and 1,2-diaminocyclohexane platinum(IV) complexes (U.S. Pat. No. 5,434,256).
Although, platinum compounds are useful against malignancies, the development of tumor resistance during the course of treatment limits their usefulness. Many of the specific antitumor mechanisms of platinum are not fully understood. Without detailed mechanistic information of platinum cytotoxicity, it has been difficult to overcome the problem of tumor resistance to cisplatin and therefore to enhance the efficacy of platinum and other anti-tumor agents.
Solid tumors contain populations of both well-oxygenated cells and hypoxic cells. Hypoxia usually occurs in cells that are farthest from the blood supply. Such cells proliferate slowly but are also relatively resistant to anti-cancer drugs. For example, data suggests that the cytotoxic effects of cisplatin are met with greater resistance by hypoxic tumor cells than by oxygenated cells (Herman et al., 1988, Cancer Research, 48:2342-2347; Melvic et al., 1988, Radiat. Res., 114:489-499; Grau et al., 1988, Radiother. Oncol., 13:301-309). Thus, one mechanism by which tumors display resistance to anti-cancer agents may be attributable to their relatively hypoxic state.
Some commonly observed side effects like renal toxicity, ototoxicity, and nephrotoxicity limit the efficacy of platinum compounds. Anemia is also a common side effect of platinum therapy. Anemia is especially common following cisplatin therapy, and patients often require blood transfusions. Although the cause of anemia following cisplatin therapy may be multifold, erythropoietin levels are found to be reduced and therefore, erythropoietin deficiency appears to be important. Recombinant human erythropoietin administered following cisplatin therapy or other chemotherapy has been reported to be effective in reversing anemia associated with such therapy (Abels 1992, Seminars in Oncology, 19:29-35).
Erythropoiesis Reaulators
The existence of a hormone that regulates erythropoiesis, the production of red blood cells, was first proposed at the beginning of the century. Since then, data has continued to mount in favor of humoral regulation of erythropoiesis. This led to the purification of erythropoietin (EPO) and determination of its amino acid sequence and ultimately to the cloning of the human EPO gene (Jacobs et al., 1985, Nature, 313:806-810; Lin, 1985, Proc. Natl. Acad. Sci., 82:7580-7584; Lin, U.S. Pat. No. 4,403,008). Purification of recombinant EPO is described in U.S. Pat. No. 4,667,016 to Lai et al.
EPO is a hormone essential in regulating levels of red blood cells in circulation. Naturally occurring EPO is produced by liver during fetal life and mainly by kidneys in adults. Recombinant erythropoietin produced by genetic engineering techniques involves the expression of a protein product from a host cell transformed with the gene encoding erythropoietin. Similar to many cell surface and secretory proteins, EPO is glycosylated. Glycosylation is usually of two types: O-linked oligosaccharides are attached to serine or threonine residues while N-linked oligosaccharides are attached to asparagine residues. Human urinary derived erythropoietin contains three N-linked and one O-linked oligosaccharides chains which comprise about 40% of the total molecular weight of the glycoprotein. Different isoforms of erythropoietin corresponding to various glycosylation levels have been described (Elliott et al., 1995, EP 0640619A1).
Erythropoietin exerts its effect by binding to the erythropoietin receptor. Activation of the EPO receptor results in several biological effects including stimulation of proliferation, stimulation of differentiation and inhibition of apoptosis (Liboi et al., 1993, Proc. Natl. Acad. Sci., USA, 1990, 11351). EPO receptor can also be activated by agonists like EPO mutants and analogs, peptides, and antibodies. In addition to EPO, other compounds with erythropoietin-like activity have also been identified. For example, a molecule identified from a renal cell carcinoma has been reported to have an EPO-like effect on erythropoiesis but is immunologically distinct from EPO (Sytkowski et al. 1979). Other stimulators of erythropoiesis include water soluble salts of transition metals (Brugnara et al., U.S. Pat. No. 5,369,014).
While platinum therapy is becoming more acceptable for advanced solid tumors, clinical correlates between hematocrit or hemoglobin and the anti-tumor effect of chemotherapeutic agents has not been evaluated to determine if agents that enhance hematocrit increase the sensitivity of tumors to anti-cancer agents. Given the increasing use of non-conventional anti-cancer agents, there is a continuing need to enhance the antitumor response to these agents.